If this suffered a hard impact, I'd do a pretty thorough inspection for physical damage before moving on. Look for loose parts in the case or any large components (like capacitors on the power supply) which are damaged or dislodged. Check the RAM and sockets to make sure they're in good shape too.
If all looks good physically, I'd move on to testing the individual power supply voltage rails to make sure power is actually good everywhere it needs to be. There could be a short somewhere from the impact damage which could be causing problems.